But what's largely gone unreported is that the bill also would lift a longstanding prohibition against primary spending by political party caucuses. That would allow committees run by current lawmakers to back state House or Senate candidates of their choosing - including themselves or their colleagues.

Tea party activists say that the proposed change would protect "establishment" candidates from grassroots challengers. Some are derisively referring to the bill as the "incumbent protection act."

"The intent is to keep the ruling political class the ruling political class," said Joan Fabiano, founder of Grassroots in Michigan. "Whoever is going to be the team player, according to the powers that be, will have considerably more financial resources at their disposal."

Party caucus committees typically rank among the biggest spenders in Michigan House and Senate general-election races. The House Republican Campaign Committee was the state's largest political action committee in the 2012 election cycle, spending $2.9 million in various races, according to the Michigan Campaign Finance Network. The House Democratic Fund was second at $2.3 million.

The caucuses were barred from spending in primary elections when they were formed nearly two decades ago. But supporters of the Senate bill, including Majority Leader Randy Richardville, have questioned why that is the case. The Monroe Republican previously pledged to defend his members against any primary challenges.

"Those members currently eligible for reelection are his colleagues and he thinks they work hard to represent their constituents and they should continue to have the opportunity to again appear on the ballot," said spokesperson Amber McCann, noting that Richardville's pledge applied to all Senate Republicans, including those who might align with the tea party.

"He believes the caucus should have the opportunity to defend its members, just as any other entity has the opportunity to support a candidate."

Democratic caucuses also would be allowed to spend in primary elections under the bill, but all 12 Senate Democrats voted against the measure, and spokesman Bob McCann said the caucus is opposed to the proposal in its entirety.

"The only people in Michigan arguing for more money in campaigns, primaries or otherwise, are the Republicans and their special interest funders," he said.

Six Senate Republicans also voted against the bill, which now sits before the House Committee on Elections and Ethics. Speaker Jase Bolger, R-Marshall, is generally interested in lifting the ban on caucus spending in primaries, according to spokesperson Ari Adler, but he is willing to listen to arguments against doing so.

"Overall, he believes it is inappropriate that party caucuses are the only ones not allowed to participate in campaigns and that it should be up to the people managing the funds to make the decision on how best to use them," Adler explained.

Government watchdog groups have criticized the bill because it would double existing contribution limits and protect the anonymity of donors that help fund electioneering ads masquerading as issue advocacy. Supporters say the limits have not been updated since 1977 and argue that legitimate issue ads are a form of free speech that should not be impinged upon.

Tea party leaders interviewed by MLive offered mixed opinions on issue ad disclosure and contribution limits, but the loosely-organized community appears to be relatively unified in its opposition to caucus spending in primaries, seen as the latest shot in what some have likened to a "civil war" within the Republican Party.

Todd Courser, who narrowly lost his 2012 bid to become Michigan GOP chairman, said the bill is offensive to tea party activists and other conservative candidates who may choose to run on the Republican ticket.

"Activists are always concerned when it looks like the establishment or party elite are moving to give themselves greater tools or power to influence the process," said Courser, who has been encouraged to run for lieutenant governor or the state House but has not yet made any decisions.

"Primaries allow the base to discipline the party leadership, and now you have a situation where leadership is going to give itself the ability to take an active role in funding primary campaigns."

Wes Nakagiri, a tea party leader who already announced his own plans to challenge Lt. Gov. Brian Calley at next year's party convention, suggested the bill could also have a chilling effect on lawmakers, who may be less inclined to oppose bills backed by their party.

"The only reason, in my view, that leadership would want to do that is to exert greater control over rank-and-file members," said Nakagiri. "If you don't toe the line, you're not going to get support in the primary, or they'll get somebody to run against you. It certainly is a step backward if you're a fan of freedom and vigorous contests."

Jonathan Oosting is a Capitol reporter for MLive Media Group. Email him, find him on Google+ or follow him on Twitter.